In accordance with various prior art teachings, such as U.S. Pat. No. 4,049,515 which issued Sept. 20, 1977, to Robinson et al., an article which appeared in JETP Letters 21, 375, in March, 1975, by Ambartzumian, et al., and disclosure of previoiusly classified information in "Laser Focus", July 1976, pages 8 and 9, it is known that a powerful infrared laser tuned to a fundamental resonance of a polyatomic molecule such as SF.sub.6, SiF.sub.4 or BCl.sub.3 can excite those molecules directly to decomposition; and furthermore in that process enrich an isotope of said molecule. The enrichment occurs because the absorption bands of all isotopes except the pumped isotope of the compound are sufficiently displaced in resonant frequency so as not to be affected by the powerful laser radiation.
Infrared radiation, tuned to a resonant frequency of a molecule, usually will excite the molecule by a single step or quanta. Additional quanta (photons) will usually not be absorbed because the molecular anharmonicity causes the excited molecule to be out of resonance with the excitation frequency. It is, of course, obvious that an additional IR laser resonant with the next level of absorption could theoretically by employed. This would, however, require an undue number of IR lasers. If the initial laser intensity, however, is sufficiently great, a phenomenon known as "power broadening" can help to overcome the disparity in resonance frequencies between the excited and unexcited molecules and provide a means for multiphoton excitation and molecular decomposition. An understanding of this process in molecules is not complete at the present time, but it is known that "power broadening" in itself is not sufficient to explain the process and that the molecules must have a high density of states, approaching a continuum, at energies above the level to which several quanta have been absorbed in order to have the multiphoton absorption continue to decomposition or to a level where chemical reaction with a second compound can occur. Robinson postulated that such a quasi-continuum could be useful and he suggested that this restricted such a multiphoton process to molecules containing four or more atoms. However he considered the process useful for isotope separation only for systems free of hot bands.
The major difficulty in applying the multiphoton absorption process with a single laser to compounds where the isotopic frequency separation is small, and/or where the absorption bands overlap, is that the laser intensity that is required to produce conversion also will produce "power broadening" that will result in a diminished isotopic selectivity. For example, in UF.sub.6, OsO.sub.4 or WF.sub.6, a CO.sub.2 laser with the intensity used by Ambartzumian, et al., or Robinson, et al., would substantially reduce selectivity.